Those skilled in the art know that spin transfer can be used as an alternative to, or in addition to, an external magnetic field in switching current perpendicular to plane (CPP) configurations of magnetic elements, otherwise known as ST-RAM cells, which may be either of the magnetic tunnel junction (MTJ) type or of the spin valve (SV) type. When a spin-polarized current passes through a free magnetic layer of a magnetic element, a portion of the spin angular momentum of the electrons incident on the free layer is transferred to the free layer. A spin transfer effect, that is caused by conduction electrons traveling from a reference magnetic layer of the magnetic element to the free magnetic layer, switches the magnetization orientation of the free layer from antiparallel, with the magnetization orientation of the reference layer, to parallel, with the magnetization orientation of the reference layer; and, a spin transfer effect that is caused by conduction electrons traveling in the opposite direction, switches the magnetization orientation of the free layer from parallel with the magnetization orientation of the reference layer back to antiparallel.
The free magnetic layer should be thick enough, compared with the spin diffusion length (SDL), to allow for efficient spin torque transfer from the spin polarized current passing therethrough. For a free magnetic layer formed by a NiFeCo alloy, the SDL for efficient spin torque transfer is on the order of 100 nm, yet a typical thickness of this free layer is approximately 5 nm or less. Those skilled in the art will appreciate that increasing a thickness of this free layer is not a desirable option, since the switching current requirement is proportional to the thickness of the layer; in addition, a thicker free magnetic layer creates a larger fringe magnetic field that can interfere with neighboring elements, particularly in a densely packed memory system. Thus, the present disclosure pertains to magnetic elements, or ST-RAM cells, having configurations facilitating a relatively dense packing of the elements and a relatively low switching current for the elements.